The infrared spectrum covers a range of wavelengths longer than the visible wavelength, but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The infrared wavelengths extend from 0.75 micrometers to 1 millimeter. The function of an infrared detector is to respond to the energy of a wavelength within some particular portion of the infrared region.
All materials generate radiant energy having characteristic wavelengths within the infrared spectrum depending upon the temperature of the material. Many current infrared image detection systems incorporate arrays with large numbers of discrete, highly sensitive detector elements, the electrical outputs of which are connected to signal processing circuitry. By analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and track sources of infrared radiation.
Infrared detector arrays which have more than a single spectral response are known. These arrays typically shift the image to separate detector elements which have the desired spectral responses. In this manner separate detector elements are used to sense separate portions of the infrared spectrum. Such infrared detector arrays require electro-mechanical and optical mechanisms to shift the image from one portion of the array to another. They also require arrays having a significantly large surface area to accommodate the detector elements needed for each of the desired spectral responses.
As such, although the prior art has recognized the problem of fabricating a compact infrared detector array having two separate spectral responses, the proposed solutions have to date been ineffective in providing a satisfactory remedy.